Date: Mon, 17 Aug 92 16:58:01 EDT
From: DJ Delorie <dj AT ctron DOT com>
To: ronis AT ronis DOT chem DOT mcgill DOT ca
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: Herculese Memory

>I have some C-code which writes directly to screen memory on a
>Hercules card.

>Specifically, through a pointer initilized as

>	unsigned char *screen = (unsigned char *)0xb8000000L;

>The code works under MSC, but fails under GCC.  The error is a
>segmentation violation, which I more or less expected; is there a
>simple fix?  (As is probably obvious, I'm not really familiar with
>programming in protected mode).

The above code is a VERY nonportable cast, and won't work in anything
other than a 16-bit DOS compiler.  To get to a pointer to a seg/ofs
address, use this formula:

	char *ptr = (char *)(0xe0000000 + seg * 16 + ofs);

Theoretically, the MK_FP() function should be used, but gcc doesn't
have one (but should).

>P.S.  I reacently asked if the -O2 option is supported in GCC under
>MSDOS, but go no reply.  Anyone know?

It is.  Read the .tex files that come with djgcc222.zip.